Offset corrugated sandwich construction

ABSTRACT

A sandwich construction is disclosed that provides alternating offset and jacent corrugated strips secured between upper and lower face sheets. The peaks of one strip are adjacent the troughs of an adjacent strip. The peaks and troughs can have flattened portions and the face sheets can be secured to said portions.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

The present invention relates generally to sandwich constructionssuitable for use in structural and non-structural members, and moreparticularly to sandwich constructions of low-density cores bondedbetween thin-gauge, high-strength face sheds for use in a wide range ofmilitary, industrial and commercial components.

Sandwich constructions are finding broad application as structural andnon-structural members in boats, aircraft, furniture, appliances, andother items requiring high strength, light-weight materials. Theytypically include face sheets bonded to either side of a low densitycore and can be made of such materials as metals, plastics or compositefiberglass laminates for rigidity and strength. Among the various corematerials are rigid plastic foams, honeycombs, truss-core corrugationsand an arrangement of adjacent, inclined rectangular-shaped facetsconnected by intermediate rows of adjoining parallelagram-shaped facets.

In aircraft design the need for weight efficiency and aerodynamicallysmooth surfaces under high stress levels has stimulated use of sandwichconstruction as a possible substitute for the conventionalsheet-stringer construction. Notwithstanding the superiorweight-efficiency of the sandwich constructions, their generalacceptance for critical applications has lagged behind theoreticaldevelopment primarily because of maintenance problems experienced inservice. Honeycomb core sandwich constructions, for instance, areconsidered to be among the most weight-efficient but, due to theirtendency to retain moisture which may become entrapped due toineffective edge seals or unbonding of face sheets from the core. Thismay degrade the structural integrity and lead to extensive corrosion andpremature failure of components. Experience has shown a significantfrequency of repairs on replacements which are very time-consuming andexpensive.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a sandwichconstruction having a high strength-to-weight efficiency suitable for abroad variety of military, industrial and commercial applications.

Another object is to provide a low-density core sandwich construction inwhich corrosion and degradation of structural integrity due to retentionof moisture is substantially eliminated.

Another object is to provide a sandwich construction in which unbondingof face sheets from the core is substantially reduced or eliminated whenheat, pressure or vacuum is applied for purposes of repair.

Still another object is to provide a sandwich construction having aflexible core with structural discontinuities for improved tolerance fordamage.

These and other objects of the invention are achieved in a sandwichconstruction comprising strips of corrugations arranged adjacent oneanother and alternately offset so that a series of peaks is next to aseries of troughs, and vice versa, and contained between upper and lowerface sheets. By arranging the strips in this alternating fashion, aplurality of passage-ways from cell to cell is provided. The outer facesheets and the strips are cocured in one operation without any secondarybonding.

The novel features which are believed to be characteristics of theinvention, both as to its organization and methods of operation,together with further objects and advantages thereof, will be betterunderstood from the following descriptions in connection with theaccompanying drawings in which the presently preferred embodiments ofthe invention are illlustrated by way of examples. It is to be expresslyunderstood, however, that the drawings are for purposes of illustrationand description only and are not intended as a definition of the limitsof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a partially cutaway, perspective view of a prior artcorrugated sandwich construction;

FIG. 2 shows a similar view of the instant invention, showing offsetcorrugationns that provide moisture pathways and also showing a seriesof shape-mandrels, extended from inside the corrugations, that can beused to form the offset pattern;

FIG. 3 shows a front elevation of the offset corrugation construction;

FIG. 4 shows a top view, with the upper face sheet partially cut away ofthe offset corrugations; and

FIG. 5 shows a cross-sectional view of the offset corrugations, takenalong lines V-V of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings wherein like referenced charactersdesignate like or corresponding components throughout the several views,there is shown in FIG. 1 a typical prior art corrugated sandwichconstruction 20 having open cells 23a,b and c alternately adjacent opencells 24a,b, contained between face sheets 21a and b. The onlydifference between the cells being that cells 23a,b and c have a flatroof section 26 adjacent face sheet 21a and cells 24a,b have a flat roofsection 27 adjacent face sheet 21b. Cells 23 and 24 are formed bycorrugations, consisting of peaks and troughs, in a core material, whichcan be various metals, plastics, composites or non-composites, and facesheets 21a and b are bonded to appropriate roof sections 26 and 27,respectively. Under stress, the face sheets may crack or separate fromthe cells, permitting moisture to accumulate therein, and with time,cause the core material to increase in weight and corrode. The usualprocedure for repair entails cleaning out any damage and resealing theface sheets by applying heat and pressure (or vacuum) thereto. This isunreliable and sometimes causes further problems with the face sheets.

Referring now to the inventive embodiment of FIGS. 2 through 5, there isshown a sandwich construction 35 comprising corrugated strips 41 and 43,each having peaks 45 and troughs 47, offset, or out of phase by apredetermined amount, contained between upper and lower face sheets 51aand b, respectively. This offset corrugated construction provideshexagonally shaped cells 55 surrounded on either side by individualtriangular cells 58. The material used for a particular core isseparated into strips 41, of approximately one-half inch in width andwrapped around a set of mandrels 60,61, as shown in FIG. 2. Strips 41can be of any suitable size and can depend on the overall size andstrength of the sandwich construction. Mandrels 60,61 maintain thedesired shape and spacing during attachment of face sheets 51a and b,but are then removed after the cocuring operation. This design allowspathways for any moisture that may find its way into the core. Mandrels60,61 can be any solid type material, such as TFE or other similarmaterials, to facilitate removal after curing.

Experimental work with a test sample constructed with this offsetcorrugated core, as compared to a sample constructed using a prior artdesign, shows a 56% increase in transverse shear stiffness in the weakerdirection (in the direction of corrugation). A comparison of bendingstiffness shows no measurable difference between the two.

Finally, while the offset corrugated core sandwich construction has beendescribed with reference to a particular embodiment, it should beunderstood that the embodiment is merely illustrative as there arenumerous variations and modifications which may be made by those skilledin the art. Thus, the invention is to be construed as being limited onlyby the spirit and scope of the appended claims.

What I claim is:
 1. A sandwich construction comprising:offset corrugatedmeans having a plurality of substantially parallel cells, each cellbeing formed by alternating peaks and troughs and having moisturecommunication with each other adjacent cell; and face sheet meanssecured to opposite sides thereof.
 2. A sandwich construction asdescribed in claim 1 wherein each said peak is substantially flat.
 3. Asandwich construction as described in claim 2 wherein each said troughis substantially flat.
 4. A sandwich construction as described in claim3 wherein said face sheet means is attached to the offset corrugatedmeans at the flat peaks and troughs.
 5. A sandwich constructioncomprising:adjacent corrugated strips arranged to have peaks of a firststrip substantially next to troughs of a second strip to form an offsetpattern; and upper and lower face sheets secured thereto.
 6. A sandwichconstruction as described in claim 5 wherein said corrugated strips aresubstantially one-half inch wide.